System and method for controlled expansion valve adjustment

ABSTRACT

A method for controlling temperature pulldown of an enclosure with a refrigeration system having a compressor, a heat rejecting heat exchanger, an expansion valve, and an evaporator comprises circulating a refrigerant through the refrigeration system, sensing a parameter of the enclosure, determining a desired evaporator pressure based upon the parameter sensed, and adjusting the expansion valve as a function of the desired evaporator pressure.

BACKGROUND OF THE INVENTION

The present invention relates generally to refrigeration systems. Moreparticularly, the present invention relates to transcriticalrefrigeration systems configured to improve temperature pulldown aftersystem start-up.

In a typical refrigeration system that utilizes a circulatingrefrigerant, the refrigerant is circulated throughout a particularrefrigerated area to remove heat from that area. The refrigerant entersthe evaporator as a liquid or as a saturated mix of liquid and vapor andthe liquid is evaporated (i.e., it boils off to pure vapor) as itabsorbs heat from the refrigerated area. This process takes place at arefrigerant temperature somewhat below the temperature of therefrigerated area in order to facilitate heat transfer from the area tothe refrigerant. The flow of refrigerant through the evaporator isnormally regulated to maintain the temperature of the vapor exiting theevaporator at some fixed margin, or “superheat,” above the saturatedtemperature of the liquid-vapor mix. This assures that exactly enoughrefrigerant is circulated to match the heat load of the refrigeratedarea. Because the refrigerated area may not require constant cooling,the refrigeration system may be turned off for a period of time, therebyallowing the refrigerated area and the refrigerant to warm to atemperature at or near the ambient temperature. When the refrigeratedarea once again requires cooling, the refrigeration system is turned on,and the refrigerant will initially go through the process of evaporationat a temperature somewhat below the ambient temperature. As therefrigerated area is cooled, the temperature of the evaporatingrefrigerant will drop accordingly until the refrigerated area reachesthe desired temperature and the system stabilizes again. The process ofcooling a refrigerated area from a warmer temperature following a systemshutdown to a desired cooler setpoint temperature is known as“pulldown.”

Refrigerants containing chlorine have been phased out in most of theworld due to their ozone destroying potential. Hydrofluorocarbons (HFCs)have been used as replacement refrigerants, but these refrigerants alsohave high global warming potential. “Natural” refrigerants, such ascarbon dioxide, have recently been proposed as replacement fluids.Unfortunately, there are problems with the use of these naturalrefrigerants as well. In particular, carbon dioxide has a low criticaltemperature, which causes the evaporator temperature and pressure to beabove the critical point and in the supercritical region during start-upof the refrigeration system. When the refrigerant is at a temperatureabove the critical temperature, there are no separate liquid and vaporphases and so the normal process of evaporation cannot take place. Whenthe evaporator temperature is supercritical there is no such thing as“superheat,” and therefore, the flow regulating device is unable tooperate properly. As a result, it becomes very difficult to control theinitial pulldown process that is necessary to bring the refrigeratedarea to the desired setpoint temperature and to return the refrigerantto a normal subcritical process.

Thus, there exists a need for a refrigeration system with improvedpulldown control when a transcritical refrigerant, such as carbondioxide, is used in a transcritical mode to provide cooling.

BRIEF SUMMARY OF THE INVENTION

The present invention is a system and method for controlling temperaturepulldown of a refrigerated enclosure with a refrigeration system havinga compressor, a heat rejecting heat exchanger, an expansion valve, andan evaporator. The method comprises circulating a refrigerant throughthe refrigeration system, sensing a parameter of the enclosure,determining a desired evaporator pressure based upon the parametersensed, and adjusting the expansion valve as a function of the desiredevaporator pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a refrigeration system according tothe present invention.

FIG. 2 is a process flow diagram illustrating the steps executed inperforming a temperature pulldown method according to the presentinvention.

FIG. 3 illustrates a graph relating pressure to enthalpy for therefrigeration system of FIG. 1 after system start-up and prior toapplication of the temperature pulldown method.

FIG. 4 illustrates a graph relating pressure to enthalpy after start-upand a first application of the temperature pulldown method.

FIG. 5 illustrates a graph relating pressure to enthalpy after start-upand a second application of the temperature pulldown method.

FIG. 6 illustrates a graph relating pressure to enthalpy after start-upand during steady-state operation after pulldown.

DETAILED DESCRIPTION

FIG. 1 illustrates a schematic diagram of refrigeration system 20, whichincludes compressor 22, gas cooler 24, expansion valve 26, evaporator28, evaporator sensor 30, enclosure sensor 31, and valve controller 32.Compressor 22 may comprise any type of compressor including, but notlimited to, reciprocating, scroll, screw, rotary vane, standing vane,variable speed, hermetically sealed, and open drive compressors.

Refrigeration system 20 is useful wherever a cooling source is needed,such as in temperature control units for buildings and automobiles.However, refrigeration system 20 will be described generically inreference to an “enclosure” that requires cooling. For example, the“enclosure” may be an office area in a building or the food storage areain a refrigerated-type food transport vehicle.

As shown in FIG. 1, refrigerant path 34 is formed by connection of thevarious elements in refrigeration system 20. Refrigerant path 34 iscreated by a loop defined by the points 1, 2, 3, and 4. After start-upof refrigeration system 20 from a non-operational mode to anoperational, cooling mode, refrigerant is first compressed withincompressor 22. The refrigerant then exits compressor 22 at high pressureand enthalpy (point 2) and is directed through gas cooler 24. Therefrigerant loses heat in gas cooler 24, and exits gas cooler 24 at lowenthalpy and high pressure (point 3). Next, the refrigerant exiting gascooler 24 is throttled in expansion valve 26. Expansion valve 26 ispreferably an electronic expansion valve (EXV). After going through anexpansion process within expansion valve 26 (point 4), the refrigerantis directed toward evaporator 28. After being heated in evaporator 28(point 1), the refrigerant once again enters compressor 22, and thecycle repeats.

As shown in FIG. 1, evaporator 28 of refrigeration system 20 is disposedwithin enclosure 36, which represents an area that requires cooling. Acirculation element 38, such as a fan or blower, is coupled to enclosure36 and is configured to direct streams of air 40 past evaporator 28 inan attempt to cool interior 42 of enclosure 36.

During initial start-up of refrigeration system 20, temperature T1 ofevaporator 28 will be approximately equal to temperature T2 of enclosure36. In particular, if refrigeration system 20 has been in thenon-operational mode for an extended period of time, it is likely thattemperatures T1 and T2 are substantially equivalent to the ambient airtemperature outside enclosure 36. When using standard, HFC refrigerants,the fact that temperature T1 of evaporator 28 may be equal to theambient temperature is not much of a concern because HFC refrigerantstypically have high critical temperatures. As a result, refrigerationsystems using HFC refrigerants tend to run “subcritical.” Systemoperation and cooling capacity are relatively easy to control in asubcritical system due to the defined relationship between pressure andtemperature in the subcritical region.

On the other hand, when using transcritical refrigerants such as carbondioxide, the fact that temperature T1 of evaporator 28 may be equal orclose to the ambient temperature is problematic because carbon dioxidehas a relatively low critical temperature. The critical temperature ofcarbon dioxide is about 87.8 degrees Fahrenheit. In warm climates, it iscommon for the ambient air temperature to exceed the criticaltemperature of carbon dioxide. When this occurs, temperatures T1 and T2may exceed the critical temperature, thus resulting in a “supercritical”evaporator temperature. As will be discussed in more detail to follow,in order to achieve effective heat transfer between evaporator 28 andenclosure 36 in such an environment, temperature T1 of evaporator 28must be decreased to a subcritical temperature, i.e., a temperature thatis below the critical temperature of the refrigerant. If temperature T1remains supercritical during operation of refrigeration system 20, thesystem will have minimal cooling capacity and, as a result, it will bedifficult or impossible to pull down the temperature of enclosure 36much below the ambient temperature. This is especially detrimental whenrefrigeration system 20 is used in, for example, a refrigeration-typetruck carrying perishable goods within enclosure 36. In that embodiment,it is critical that refrigeration system 20 is capable of pulling downtemperature T2 of enclosure 36 to a low temperature within a shortamount of time so that the perishable goods do not spoil. However,without having the capability to pull down temperature T1 f evaporator28 into the subcritical region, refrigeration system 20 is almostuseless as a cooling source. The present invention provides a system andmethod for operating a refrigeration system to pull down an enclosuretemperature while operating in either a subcritical or a supercriticalcycle.

In refrigeration system 20, expansion valve 26, evaporator sensor 30,enclosure sensor 31, and valve controller 32 operate together to enablesufficient enclosure temperature pulldown such that refrigeration system20 remains useful as a cooling source even when operating in anenvironment wherein the ambient temperature is above the criticaltemperature of the refrigerant. Evaporator sensor 30 of refrigerationsystem 20 is coupled to evaporator 28, and is configured to sense aparameter within evaporator 28 and send a signal corresponding to theparameter to valve controller 32. Preferably, the parameter sensed byevaporator sensor 30 is evaporator pressure, although other parameters(such as temperature) that may be sensed and used to deduce pressure arealso contemplated. Similarly, enclosure sensor 31 of refrigerationsystem 20 is coupled to enclosure 36, and is configured to sense aparameter within enclosure 36, such as temperature, and send a signalcorresponding to the parameter to valve controller 32. Valve controller32 may use a combination of, for example, the evaporator pressure,enclosure temperature, and the desired enclosure temperature setpoint todetermine a desired evaporator pressure that will reduce the evaporatortemperature to a subcritical temperature and enable pulldown of theenclosure temperature to the desired temperature setpoint.

In one embodiment of the present invention, enclosure sensor 31 includesa temperature transducer such as a thermocouple, RTD (resistancetemperature detector), or thermistor. Enclosure sensor 31 is configuredto sense the temperature within interior 42 of enclosure 36 and send asignal to valve controller 32. Based upon the enclosure temperature,valve controller 32 determines the proper adjustment to the evaporatorpressure necessary in order to attain the requisite heat transferbetween evaporator 28 and enclosure 36 and achieve the desired enclosuresetpoint temperature.

Furthermore, in one embodiment, expansion valve 26 is an electronicexpansion valve (EXV) and evaporator sensor 30 includes a pressuretransducer embedded in an evaporator tube to measure the refrigerantpressure. The pressure transducer provides a feedback signal to valvecontroller 32 which accordingly controls the movement of expansion valve26. The EXV includes is a mechanical valve coupled to a stepper motor tocontrol the opening and closing of the valve orifice. The stepper motorresponds to the valve controller input by opening or closing the valveorifice as necessary. Typically, the pressure drop is modified bycontrolling the size of an orifice or flow restriction disposed withinexpansion valve 26.

For normal steady-state operation where the evaporator is in asubcritical state, evaporator sensor 30 may additionally include atemperature transducer in order to determine superheat of therefrigerant vapor exiting evaporator 28 by comparing the temperature ofthe vapor to the saturated pressure within evaporator 28.

FIG. 2 is a process flow diagram of a method 50 for controllingtemperature pulldown of an enclosure with a refrigeration system. Forpurposes of example, method 50 will be discussed in reference torefrigeration system 20 of FIG. 1.

Method 50 begins at step 52 by circulating a refrigerant through arefrigeration system, such as refrigeration system 20. Method 50continues at step 54 by sensing a parameter of an enclosure thatrequires cooling. In one embodiment of the present invention, the sensedparameter is the temperature of enclosure 36. Next, in step 56, adesired evaporator pressure is determined based upon the sensedparameter within the enclosure. Any parameter or combination ofparameters that enables refrigeration system 20 to determine the desiredevaporator pressure is within the intended scope of the presentinvention. Then, in step 58, the expansion valve is adjusted as afunction of the desired evaporator pressure. In one embodiment,expansion valve 26 is adjusted to lower the evaporator pressure from asupercritical pressure to a subcritical pressure. After adjusting theexpansion valve in step 58, an actual evaporator pressure is determinedin step 60, such as with evaporator sensor 30. Finally, in step 62, theexpansion valve is adjusted as a function of the actual evaporatorpressure determined in step 60. It is important to note than in someinstances, it may be necessary to perform steps 54-62 continuously or atdefined intervals, as indicated by arrow 64, in order to achieve ormaintain the desired enclosure setpoint temperature.

In some instances, the various steps comprising method 50 may beperformed in a slightly different order. Furthermore, one or more of thesteps may be omitted without departing from the intended scope of thepresent invention. For example, steps 60 and 62 may be omitted such thatmethod 50 adjusts the expansion valve based solely on sensing theenclosure parameter and not on the actual evaporator pressure as well.

By performing method 50, it is possible to pull down the enclosuretemperature in a refrigeration system that utilizes any type ofrefrigerant, operating in either subcritical or transcritical cycles.However, method 50 is particularly useful in conjunction withrefrigeration systems configured to operate in a transcritical mode. Asdiscussed previously, these types of systems typically run supercriticalwhen used in a hot ambient temperature. The system and method of thepresent invention enables pulldown of the enclosure temperature even inhot ambient conditions. Thus, the present invention allows arefrigeration system to maintain the evaporator in a subcritical stateeven when operating in an environment above the critical temperature ofthe refrigerant being used.

FIG. 3 illustrates a graph relating pressure to enthalpy after start-upof refrigeration system 20 and prior to application of temperaturepulldown method 50. As shown in FIG. 3, refrigeration system 20 isconfigured to circulate carbon dioxide. However, it should be understoodthat carbon dioxide is used merely for purposes of example and not forlimitation. Furthermore, the cycle in FIG. 3 assumes that the heatexchangers in refrigeration system 20 are ideal and that the pressurewithin evaporator 28 is held substantially constant.

In FIG. 3, vapor dome V is formed by a saturated liquid line and asaturated vapor line, and defines the state of the refrigerant atvarious points along the refrigeration cycle. Underneath vapor dome V,all states involve both liquid and vapor coexisting at the same time. Atthe very top of vapor dome V is critical point P. The critical point Pis defined by the highest temperature and pressure where saturatedliquid and saturated vapor coexist. In general, compressed liquids arelocated to the left of vapor dome V, while superheated vapors arelocated to the right of vapor dome V. As critical point P is approached,the properties of both liquid and gas become the same. Thus, above thecritical point, there is only one phase. In particular, above itscritical pressure, a substance cannot be separated into liquid and vaporphases.

As shown in FIG. 3, within vapor dome V the temperature of therefrigerant remains constant at a specified pressure. Thus, the pressureand temperature of a refrigerant in the subcritical region are directlyrelated. However, outside of vapor dome V, there is no specificrelationship between temperature and pressure. For example, the pressurewithin evaporator 28 (between points 4 and 1) remains around 1200 psia,but the temperature within evaporator 28 increases from about 85 degreesFahrenheit (point 4) at the inlet of evaporator 28 to about 100 degreesFahrenheit at the outlet (point 1). Therefore, outside of thesubcritical region of vapor dome V, the relationship between temperatureand pressure disappears.

In FIG. 3, refrigerant path 34 is the loop defined by the points 1, 2,3, and 4. The cycle begins in the main path at point 1, where therefrigerant is a low pressure, high enthalpy supercritical fluid priorto entering compressor 22. After compression within compressor 22, therefrigerant exits compressor 22 at high pressure and enthalpy, as shownby point 2. Then, as the refrigerant flows through gas cooler 24,enthalpy decreases while pressure remains constant, and the refrigerantexits as a cooler supercritical fluid. After exiting gas cooler 24, therefrigerant is then throttled in expansion valve 26, decreasing pressureas shown by point 4. Finally, the refrigerant is directed throughevaporator 28, where it exits as a higher enthalpy supercritical fluidas shown by point 1. As shown in FIG. 3, points 1, 2, 3, and 4 of therefrigeration cycle reside above the critical point P. When every pointof a refrigeration cycle is located above the critical point for therefrigerant, the cycle is known as a “supercritical” cycle. In thissupercritical region, the liquid and gas phases are no longer clearlydistinguishable from each other, and the refrigerant remains asupercritical fluid throughout the entire cycle.

In a refrigeration system, the specific cooling capacity, which is themeasure of total cooling capacity divided by refrigerant mass flow, maytypically be represented on a graph relating pressure to enthalpy by thelength of the evaporation line. As shown in FIG. 3, the specific coolingcapacity of refrigeration system 20 after start-up is represented by thelength of evaporation line L from point 4 to point 1. The specificcooling capacity determines the amount of heat transfer possible betweena refrigeration system and an area to be cooled. In particular, thelocation of point 1 along evaporation line L is directly related to thetemperature at point 1 which in turn is generally proportional to thetemperature of the area to be cooled. Note that with an increase inpressure, the constant temperature lines near point 1 curve towards theleft. Therefore, for a given enclosure temperature, with an increase inpressure, the maximum possible specific capacity decreases as point 1slides left along the constant enclosure temperature isotherm. Also, fora given enclosure temperature, an increase in pressure causes theevaporator temperature to increase, thereby decreasing the availabletemperature differential between the enclosure and the evaporator, anddecreasing the heat transfer between the refrigerant and the enclosure.As a result, there is an adverse effect on the specific capacity.

In FIG. 3, both the enclosure temperature E1 of enclosure 36 and theambient temperature A of the air outside enclosure 36 are about 100degrees Fahrenheit. Furthermore, in this example, the desiredtemperature setpoint D of enclosure 36 is approximately 30 degreesFahrenheit. Thus, in order to cool enclosure 36 to desired setpoint D,refrigeration system 20 must have sufficient cooling capacity. Inparticular, what drives the heat exchange between evaporator 28 andenclosure 36 is the temperature difference ΔT1 between evaporator 28 and20 enclosure 36. As shown in FIG. 3, temperature difference ΔT1 is about15 degrees Fahrenheit at point 4 and decreases rapidly to about 0degrees Fahrenheit at point 1. Due to the small temperature difference,the cooling capacity of the system is also small. Therefore, it is verydifficult to pull down the temperature within enclosure 36 to desiredtemperature setpoint D (especially in a short period of time) withoutadjusting expansion valve 26, such as by method 50 as discussed above.

FIG. 4 illustrates a graph relating pressure to enthalpy after start-upand a first application of temperature pulldown method 50. As shown inFIG. 4, the adjustment of expansion valve 26 has caused the pressure ofevaporator 28 to drop below vapor dome V and into the two-phasesubcritical region of the vapor dome. In particular, the evaporatorpressure has dropped from about 1200 psia to about 700 psia, while thegas cooler pressure has remained constant at about 1600 psia. After thefirst application of method 50, points 2 and 3 of the refrigerationcycle remain above vapor dome V, while points 1 and 4 now reside belowvapor dome V. Whenever the gas cooler pressure is above the vapor domeand the evaporator pressure is below the vapor dome, the refrigerationcycle is known as a “transcritical” cycle.

Inside of vapor dome V, the evaporator temperature remains constant. Asa result, at a constant pressure, temperature difference ΔT2 alsoremains constant within this region. Therefore, unlike temperaturedifference ΔT1 of FIG. 3 which continuously varied even at a constantpressure, temperature difference ΔT2 is both known and constant at alltimes within vapor dome V. In particular, within vapor dome V,temperature and pressure are directly related. Therefore, in thissubcritical region, the temperature of the refrigerant determines thepressure, and vice versa. This fixed relationship allows precise controlof both evaporator temperature and pressure. Thus, a particularevaporator temperature may be achieved by adjusting expansion valve 26to the evaporator pressure that corresponds with that temperature. Inparticular, method 50 allows refrigeration system 20 to constantlymonitor and control the temperature difference between evaporator 28 andenclosure 36, and in turn, the cooling capacity of the system.

As stated above, adjusting the pressure drop caused by expansion valve26 such that the evaporator pressure is now within the subcriticalregion results in an increased refrigeration capacity. This increasedcapacity is represented by the length of the evaporation line from point4 to point 1. The main factor contributing to the increasedrefrigeration capacity is the large increase in the enthalpy at theevaporator exit temperature. As shown in FIG. 4, the evaporator capacityhas increased over the supercritical cycle of FIG. 3 even though theevaporator outlet temperature has remained the same. In addition, notonly has the refrigeration capacity increased, but the ability tocontrol the capacity has also improved by the transition from asupercritical cycle where there is no defined relationship betweentemperature and pressure to a transcritical cycle where the relationshipbetween temperature and pressure is known.

It should be noted that decreasing the evaporator pressure further for agiven enclosure temperature may not necessarily increase the capacityfurther since the lower pressure also decreases the density of the vaporreturning to compressor 22 at point 1, and thus decreases the total massflow of the circulating refrigerant. The optimal pressure in evaporator28 will be a tradeoff between the increased specific capacity, as seenby comparing the pressure-enthalpy diagrams of FIGS. 3 and 4, and thelower total mass flow resulting from the lower vapor density at point 1.Therefore, valve controller 32 must be programmed to determine theoptimal pressure in evaporator 28 for a given enclosure temperature inorder to maximize the net cooling capacity of the resulting refrigerantflow.

As shown in FIG. 4, enclosure temperature E2 is still substantiallyequivalent to ambient air temperature A after a first application ofmethod 50. This results from the fact that the evaporator pressure hasjust dropped down into the subcritical region, and there has not been asufficient amount of time for heat to transfer from enclosure 36 to therefrigerant flowing through evaporator 28. However, as will be seen inthe following figures, the system and method of the present inventionwill result in a decrease in the enclosure temperature to the desiredtemperature setpoint D over time.

FIG. 5 illustrates a graph relating pressure to enthalpy after a secondapplication of temperature pulldown method 50. As shown in FIG. 5, thecontrolled adjustment of expansion valve 26 has caused the pressure ofevaporator 28 to drop to a lower pressure within vapor dome V. Inparticular, the evaporator pressure has dropped to about 550 psia, whilethe gas cooler pressure has remained constant at about 1600 psia. Afterthe second application of method 50, the refrigeration cycle is stilloperating as a transcritical cycle. However, the pressure differencebetween the high side gas cooler pressure and the low side evaporatorpressure has increased.

As shown in FIG. 5, enclosure temperature E3 has dropped from about 100degrees Fahrenheit to about 60 degrees Fahrenheit. This decrease inenclosure temperature is a direct result of the controlled adjustment ofexpansion valve 26 according to temperature pulldown method 50. Withoutadjusting expansion valve 26 to decrease the evaporator pressure into aregion of two-phase flow, it would not have been possible to achieve thedecrease in the enclosure temperature under the present conditions.

By performing temperature pulldown method 50, refrigeration system 20has been able to pull down enclosure temperature E3 closer towarddesired temperature setpoint D, which is about 30 degrees Fahrenheit.However, since desired temperature setpoint D of enclosure 36 is lowerthan enclosure temperature E3 as shown in FIG. 5, it may be necessary todecrease the evaporator pressure even further to lower the evaporatortemperature to enable sufficient heat transfer. This may be accomplishedby once again performing temperature pulldown method 50, as will berepresented graphically in FIG. 6.

It is important to note that from a control point of view, when theenclosure temperature is reasonably below the critical temperature ofthe refrigerant, it may no longer be necessary to monitor the enclosuretemperature and evaporator temperature. A metering of refrigerant basedon the evaporator superheat may be sufficient to control the systemoperation.

FIG. 6 illustrates a graph relating pressure to enthalpy for the final,steady state operation of the system where enclosure temperature E4 issubstantially equivalent to desired temperature setpoint D afterapplication of pulldown method 50 over a reasonably short period oftime. In particular, the temperature of enclosure 36 has been pulleddown from ambient temperature A in FIG. 3 to desired temperaturesetpoint D in FIG. 6 through controlled adjustment of expansion valve26. When an enclosure temperature reaches and maintains a desiredsetpoint temperature, the refrigeration system is said to be in steadystate operation. At steady state operation, it is no longer necessary tocontrol expansion valve 26 as described previously in order to maintainenclosure temperature E4 at desired temperature setpoint D. At steadystate, refrigeration system 20 may continue to operate by application ofa method similar to method 50 described above. However, refrigerationsystem 20 may alternatively use any number of other devices and methodsto control the evaporator temperature during steady state operation ofrefrigeration system 20. For example, refrigeration system 20 mayinclude an additional sensor disposed near the outlet of evaporator 28that is configured to sense the temperature of the refrigerant flowingthrough the outlet and control temperature of the refrigerant within theevaporator based upon this sensed value.

Although the present invention has been described in reference to threeapplications of method 50 prior to reaching steady state operation,embodiments that require more or less applications of method 50 arewithin the intended scope of the present invention. In particular, thenumber of applications required depends on many factors, including thedesired efficiency, the desired time to pull down to the setpointtemperature, and the desired size of the evaporator pressure changes tomaintain effective performance during pulldown. Therefore, the presentinvention has been described in reference to three applications oftemperature pulldown method 50 for purposes of example and not forlimitation.

In addition, it should be understood that carbon dioxide was used as therefrigerant for purposes of example only. The system and method of thepresent invention may be used with any other type of refrigerant withoutdeparting from the intended scope of the present invention.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method for controlling temperature pulldown of an enclosure with arefrigeration system having a compressor, a heat rejecting heatexchanger, an expansion valve, and an evaporator, the method comprising:circulating a refrigerant through the refrigeration system; sensing aparameter of the enclosure; determining a desired evaporator pressurebased upon the parameter sensed; and adjusting the expansion valve as afunction of the desired evaporator pressure.
 2. The method of claim 1,wherein the parameter of the enclosure is temperature.
 3. The method ofclaim 1, and further comprising: determining an actual evaporatorpressure within the evaporator; and adjusting the expansion valve as afunction of the actual evaporator pressure.
 4. The method of claim 1,wherein the refrigerant operates in a transcritical refrigeration cycle.5. The method of claim 1, wherein the refrigerant is carbon dioxide. 6.The method of claim 1, wherein the step of adjusting the expansion valveas a function of the desired evaporator pressure decreases an actualevaporator pressure from a supercritical pressure to a subcriticalpressure.
 7. The method of claim 1, wherein the expansion valve is anelectronic expansion valve.
 8. A refrigeration system for cooling anenclosure comprising: a compressor for compressing a refrigerant to agas cooler pressure, wherein the gas cooler pressure is a supercriticalpressure; a gas cooler for cooling the refrigerant; an evaporator forheating the refrigerant, wherein the evaporator has an evaporatorpressure; an expansion valve disposed between the gas cooler and theevaporator and configured to reduce the pressure of the refrigerant fromthe supercritical gas cooler pressure to a desired evaporator pressure,wherein the desired evaporator pressure is a subcritical pressure; and asensor for monitoring an enclosure parameter.
 9. The refrigerationsystem of claim 8, wherein the refrigerant is carbon dioxide.
 10. Therefrigeration system of claim 8, wherein the refrigerant operates in atranscritical refrigeration cycle.
 11. The refrigeration system of claim8, wherein the enclosure parameter is enclosure temperature.
 12. Therefrigeration system of claim 8, wherein the sensor is configured tosend a signal to a valve controller indicative of the enclosureparameter.
 13. The refrigeration system of claim 12, wherein the valvecontroller is configured to adjust the evaporator pressure based uponthe enclosure parameter.
 14. The refrigeration system of claim 8,wherein the expansion valve is an electronic expansion valve.
 15. Amethod for operating a refrigeration system having a compressor, a heatrejecting heat exchanger, an expansion valve, and an evaporator, themethod comprising: circulating a refrigerant through the refrigerationsystem; and adjusting an orifice of the expansion valve as a function ofa sensed parameter to decrease an evaporator pressure from asupercritical pressure to a subcritical pressure.
 16. The method ofclaim 15, wherein the sensed parameter is temperature.
 17. The method ofclaim 15, wherein the sensed parameter is pressure.
 18. The method ofclaim 17, wherein the step of adjusting the orifice of the expansionvalve comprises sensing the evaporator pressure and comparing the 5evaporator pressure to a desired pressure.
 19. The method of claim 15,wherein the refrigerant operates in a transcritical refrigeration cycle.20. The method of claim 15, wherein a valve controller receives thesensed parameter and is configured to adjust the orifice of theexpansion valve based on the sensed parameter.